/** Add a block to the list of pending Replications */
 void add(Block block, int numReplicas) {
   synchronized (pendingReplications) {
     PendingBlockInfo found = pendingReplications.get(block);
     if (found == null) {
       pendingReplications.put(block, new PendingBlockInfo(numReplicas));
     } else {
       found.incrementReplicas(numReplicas);
       found.setTimeStamp();
     }
   }
 }